grab viewport as pdf file of custom size

I have a processing script with multiple 3D animated spheres in OpenGL. I am getting rid of the background() in order to pan or zoom the camera and have the trails of the spheres drawn on the canvas. 

So, I would like to capture the drawing created as I already described, as a PDF file. However the options that I have already tried based on the create PDF file found here: http://processing.org/reference/libraries/pdf/

do not seem to function. 

Do you have any idea what can I do? It's more or less something like a print screen but with kind of huge resolution...